Introduction to OSPF
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Transcript of Introduction to OSPF
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Introduction to OSPF
Campus Networking Workshop
These materials are licensed under the Creative Commons Attribution-Noncommercial 3.0 Unported license (http://creativecommons.org/licenses/by-nc/3.0/)
Campus Networking Workshop
Introduction to OSPF
Modified from originals by Philip Smith
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Note: Routing and Forwarding
• Routing is not the same as Forwarding • Routing is the building of maps
• Each routing protocol usually has its own routing database
• Routing protocols populate the forwarding table • Forwarding is passing the packet to the next hop
device • Forwarding table contains the best path to the next hop
for each prefix • There is only ONE forwarding table
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OSPF Background
• Developed by IETF – RFC1247 • Designed for Internet TCP/IP environment
• OSPF v2 described in RFC2328/STD54 • Link state/Shortest Path First Technology • Dynamic Routing • Fast Convergence • Route authentication
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Link State Algorithm
• Each router contains a database containing a map of the whole topology • Links • Their state (including cost)
• All routers have the same information • All routers calculate the best path to every
destination • Any link state changes are flooded across the
network • “Global spread of local knowledge”
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Link State Routing
• Automatic neighbour discovery • Neighbours are physically connected routers
• Each router constructs a Link State Packet (LSP) • Distributes the LSP to neighbours… • …using an LSA (Link State Advertisement)
• Each router computes its best path to every destination
• On network failure • New LSPs are flooded • All routers recompute shortest path tree
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Low Bandwidth Requirements
• Only changes are propagated • Multicast used on multi-access broadcast
networks • 224.0.0.5 used for all OSPF speakers • 224.0.0.6 used for DR and BDR routers
FDDI Dual Ring
R1
LSA
X LSA
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“Shortest Path First”
The optimal path is determined by the sum of the interface costs: Cost = 108/bandwidth
N1
N2 N3
N4
N5 R1
R2
R3
R4
Cost = 1 Cost = 1
Cost = 10
Cost = 10
Cost = 10
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OSPF: How it works
Hello Protocol • Responsible for establishing and maintaining
neighbour relationships • Elects Designated Router on broadcast networks
FDDI Dual Ring
Hello
Hello Hello
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OSPF: How it works
• Hello Protocol • Hello Packets sent periodically on all OSPF enabled
interfaces • Adjacencies formed between some neighbours
• Hello Packet • Contains information like Router Priority, Hello Interval,
a list of known neighbours, Router Dead Interval, and the network mask
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OSPF: How it works
• Trade Information using LSAs • LSAs are added to the OSPF database • LSAs are passed on to OSPF neighbours
• Each router builds an identical link state database
• SPF algorithm run on the database • Forwarding table built from the SPF tree
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OSPF: How it works
When change occurs: • Announce the change to all OSPF neighbours • All routers run the SPF algorithm on the revised
database • Install any change in the forwarding table
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Broadcast Networks
• These are network technologies such as Ethernet • Introduces Designated and Backup Designated
routers (DR and BDR) • Only DR and BDR form full adjacencies with other
routers • The remaining routers remain in a “2-way” state with
each other • If they were adjacent, we’d have n-squared scaling problem
• If DR or BDR “disappear”, re-election of missing router takes place
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Designated Router
One per multi-access network • Generates network link advertisements for the multi-
access network • Speeds database synchronisation
Designated Router
Designated Router
Backup Designated Router
Backup Designated
Router
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Designated Router
• All routers are adjacent to the DR • All routers are adjacent to the BDR also
• All routers exchange routing information with DR • BDR also stays synchronized with the DR
• DR updates the database of all its neighbours • BDR waits silently and only takes over if DR dies
• This scales! • 2n problem rather than having an n-squared problem.
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Designated Router
• Adjacencies only formed with DR and BDR • LSAs propagate along the adjacencies
DR BDR
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Designated Router Priority
• Determined by interface priority • Otherwise by highest router ID
• (For Cisco IOS, this is address of loopback interface, otherwise highest IP address on router)
144.254.3.5
R2 Router ID = 131.108.3.3
131.108.3.2 131.108.3.3
R1 Router ID = 144.254.3.5
DR
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More Advanced OSPF
• OSPF Areas • Virtual Links • Router Classification • OSPF route types • External Routes • Route authentication • Equal cost multipath
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OSPF Areas
• Group of contiguous hosts and networks
• Per area topological database • Invisible outside the area • Reduction in routing traffic
• Backbone area contiguous • All other areas must be
connected to the backbone
• Virtual Links Area 1 Area 4
Area 0 Backbone Area
Area 2 Area 3
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OSPF Areas
• Reduces routing traffic in area 0 • Consider subdividing network into areas
• Once area 0 is more than 10 to 15 routers • Once area 0 topology starts getting complex
• Area design often mimics typical ISP core network design
• Virtual links are used for “awkward” connectivity topologies
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Virtual Links
• OSPF requires that all areas MUST be connected to area 0
• If topology is such that an area cannot have a physical connection to a device in area 0, then a virtual link must be configured
• Otherwise the disconnected area will only be able to have connectivity to its immediately neighbouring area, and not the rest of the network
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Classification of Routers
Internal Router (IR) Area Border Router
(ABR) Backbone Router (BR) Autonomous System
Border Router (ASBR)
Area 0
Area 2 Area 3
IR
ABR/BR
To other AS
ASBR
Area 1
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OSPF Route Types
Intra-Area route All routes inside an area
Inter-Area route Routes advertised from one
area to another area by an ABR
External route Routes imported into OSPF
from another routing protocol by an ASBR
Area 0 Area 2 Area 3
ABR
To other AS
ASBR
Area 1
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External Routes
Type 1 external metric: metrics are added to the summarised internal link cost
Network N1 N1
Type 1 11 10
Next Hop R2 R3
Cost = 10
to N1 External Cost = 1
to N1 External Cost = 2 R2
R3
R1
Cost = 8
Selected Route
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External Routes
Type 2 external metric: metrics are compared without adding to the internal link cost
Network N1 N1
Type 2 1 2
Next Hop R2 R3
Cost = 10
to N1 External Cost = 1
to N1 External Cost = 2 R2
R3
R1
Cost = 8
Selected Route
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Route Authentication
Now recommended to use route authentication for OSPF …and all other routing protocols
Susceptible to denial of service attacks OSPF runs on TCP/IP Automatic neighbour discovery
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Equal Cost Multipath
If n paths to same destination have equal cost, OSPF will install n entries in the forwarding table Loadsharing over the n paths Useful for expanding links across an ISP
backbone Don’t need to use hardware muliplexors Don’t need to use static routing
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OSPFv3
• OSPFv2 only supports IPv4 • OSPFv3 developed for IPv6 only
• Dual stack networks need to run both protocols • They run independently of each other
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OSPFv2 vs. OSPFv3
• Very similar, with a few differences • New LSA types to separate links from their
prefixes • Avoids SPF recalculations when only the link prefix
changes
• Removes OSPF-specific authentication • Relies on underlying IPv6 security headers
• Supports multiple instances
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OSPF Configuration – Start
• Start the OSPF process router ospf 100
• “100” is the process ID • Process ID is unique to the router
• You can run multiple OSPF processes on the same router
• It is common to set the process ID to autonomous system (AS) number
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OSPF Configuration Advertising Networks Option 1
router OSPF 100
redistribute connected subnets
• Includes all the subnets connected to the router, but announces them as external type-2 LSAs, which are not summarized • Not a good option when using multiple areas
• It also does not give you control over which networks you want to announce
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OSPF Configuration Advertising Networks Option 2
• Per-link configuration (IOS 12.4 and later) • OSPF configured on each interface • Passive interfaces do not establish adjacencies
router ospf 100
passive-interface default
no passive interface GigabitEthernet0
interface GigabitEthernet0
ip address 10.10.0.1 255.255.255.0
ip ospf 100 area 0
interface FastEthernet0
ip address 192.168.10.1 255.255.255.0
ip ospf 100 area 0
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OSPF Configuration Advertising Networks Option 3
• Network statements • Annouce subnets included in given network blocks • Add one network statement per subnet, or use wildcard
masks to include multiple subnets
ospf router 100
passive-interface default no passive-interface GigabitEthernet0
network 10.10.0.0 0.0.0.255 area 0
network 192.168.0.0 0.0.0.3 area 0
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OSPF Configuration Authenticating adjacencies
• Very important to control which devices can establish adjacencies
• A malicious user could inject routes and steal your traffic!
router ospf 100
area 0 authentication message-digest
interface GigabitEthernet0/0
ip ospf authentication-key <key>
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Questions?